Angiotensin II has been shown to be mitogenic in different cell types. The signaling pathway by which angiotensin II promotes cell proliferation includes activation of intracellular kinase cascades and subsequent activation of transcription factors. One such pathway is the Jak STAT pathway. The tyrosine kinase Jak2 is an integral component of this pathway. In rat aortic smooth muscle (RASM) cells, angiotensin II leads to the rapid tyrosine phosphorylation and activation of Jak2. Angiotensin II also induces the physical association of Jak2 with the AT/1 receptor.. At present, the structural features of the AT/1 receptor necessary for intracellular tyrosine kinase activation are not understood. My preliminary results suggest that the carboxyl terminal 54 amino acids of the rat AT/1 receptor physically binds with Jak2 in the angiotensin II- dependent manner. Both in vitro and in vivo analysis shows that this association is dependent on the AT/1 receptor motif YIPP (amino acids 319 322). The binding of the AT/1 receptor with the intracellular tyrosine kinase Jak2 supports the concept that the seven transmembrane superfamily of receptors can physically associate with enzymatically active intracellular proteins creating a signaling complex mechanistically similar to that observed with growth factor and cytokine receptors. It was reported previously that angiotensin II stimulates the transient tyrosine phosphorylation of STAT1, STAT2 and STAT3. I now present evidence that angiotensin II mediated activation of Jak2 leads to tyrosine phosphorylation of STAT1 and this phosphorylation event is modulated by the AT/1 receptor. Furthermore, the results suggest that Jak-STAT pathway acts as link between cell surface receptors and nuclear transcriptional events leading to cell growth. Based on these observations, I intend to define the precise role of Jak- STAT pathway in angiotensin II mediate vascular smooth muscle cell growth. The goals of the proposal are 1) to define the structural feature(s) of the AT1 receptor required for Jak2 binding, 2) to define the mechanism of angiotensin II dependent STAT1 phosphorylation, and 3) to elucidate the role of Jak2 in AT/1 receptor mediated cell growth. It is anticipated that this research will help to understand the molecular mechanisms associated with vascular smooth muscle cell growth.